Membrane lipid composition and rapid endocytic retrieval modulate Wingless secretion

The Wnt family ligands carry a lipid moiety that must be shielded from the aqueous environment. During secretion, this is achieved by the cargo receptor Wntless. The effect of acute endocytosis inhibition suggests that Wingless dissociates from Wntless at the apical plasma membrane but is prevented from being released extracellularly by rapid endocytosis. Super-resolution microscopy shows that Wingless then traffics, without Wntless, through specialized Rab7/Rab4-positive endosomes before basolateral release. In the absence of endocytosis, apical release is allowed, albeit in punctae. Similar punctae are formed upon abrogation of the ceramide synthase Schlank. We suggest that they represent insoluble aggregates since they are 'solubilized' by the lipid-binding protein Dally-like protein. Moreover, purified Wingless forms similar-looking structures that are solubilised by liposomes in vitro. These findings suggest that membrane lipid composition could modulate the stability of the Wingless-Wntless complex and/or the lipid bilayer's ability to accommodate the Wingless lipid. Together with rapid endocytosis, this ensures orderly transfer of Wingless from Wntless to the membrane, preventing aggregation, and promoting basolateral secretion.